The present invention is directed to the field of staged combustion systems. Such combustion systems supply two reactants, typically fuel and air, to a burner to be combusted. In a staged burner, a first reactant is supplied in two flow streams, a primary flow and a secondary flow. The primary flow of the first reactant is combusted with the entirety of a second reactant in a primary combustion stage. The secondary flow of the primary reactant is combusted with the burnt effluent of the primary stage in a secondary combustion stage. Either fuel or oxidant can be supplied as the primary reactant. Specifically, a staged burner can be either air-staged or fuel-staged.
A typical previous fuel-staged combustion system 10 is shown in FIG. 1. Of course, those skilled in the art would appreciate that this system could also be configured as an air-staged system. In this previous system 10, an air flow 12 is supplied using a blower 14. A metering orifice plate 16 is used to create a pressure differential which defines a desired air flow rate. The fuel is supplied from a common supply 18 with a metering orifice plate 20 used to create a pressure differential which defines a desired fuel flow rate.
In the fuel-staged system shown, the common supply 18 is divided into a primary fuel flow 22 and a secondary flow 24. The primary fuel flow 22 is combusted with the air flow 12 in the primary combustion stage 26. The secondary flow 24 is combusted with the burnt effluent of the primary stage 26 in the secondary combustion stage 28, which is typically a furnace environment. The rate of the primary flow 22 is defined by a limiting orifice 30 which is adjusted to provide a desired flow to the primary stage 26. Similarly, the rate of the secondary flow 24 is defined by another limiting orifice 32 which is adjusted to provide a desired flow to the secondary stage 26. In this way the split between the two stages is controlled.
The flow rates to the primary and secondary stages are defined by the limiting orifices 30, 32 in order to provide a desired equivalence ratio .phi. to the primary stage 26 and the burner 10 overall. The equivalence ratio .phi. is related to the fuel-to-air ratio and measures the proportion of fuel to the proportion of air in a combustion reaction. The equivalence ratio is given by the following relationship: ##EQU1## where F and A respectively signify proportional reactive volumes of fuel and air. Stoichiometric burner operation is defined as .phi.=1, where fuel and air are supplied in a proportion to produce a complete combustion reaction. For .phi.&gt;1, the burner fires rich, i.e. with excess fuel. With rich firing, the fuel is not completely combusted with the available supplied air. For .phi.&lt;1, the burner fires lean, i.e. with an excess of air. With lean firing, the excess air contributes to the thermal load, diluting the heat released by combustion.
Stoichiometric firing (.phi.=1) is theoretically the most efficient burner operation since, at this ratio, the maximum heat is released by the combustion reaction. However, stoichiometric firing is difficult to maintain. Also carbon monoxide production increases near stoichiometric firing. As a practical matter, burners are typically fired slightly lean, at about 10% excess air (.phi.=0.909), an equivalence ratio which offers a good balance between efficiency and carbon monoxide production. Burners are staged to provide a desired combustion result and a equivalence ratio .phi. for the primary zone is selected such that an optimum performance by the combustion system is achieved.
In a fuel-staged system such as illustrated in FIG. 1, the primary fuel flow 22 is supplied so as to run lean in the primary stage 26, i.e. with an equivalence ratio .phi. less than 1. The additional fuel is supplied at the secondary stage 28 in order to consume the remaining air, thereby raising the overall burner equivalence ratio .phi. to about 0.909, approaching a practical efficient level of combustion. In another example, an air-staged system has a primary air flow configured so that the primary stage runs rich, i.e. with an equivalence ratio .phi. greater than one. With such stoichiometry, the reaction in the primary stage is incomplete. Secondary air is supplied in the secondary stage in order to complete the reaction, reducing the overall burner equivalence ratio to about 0.909.
Staged burners have several advantages over conventional single-stage burners. By combusting the fuel in two stages, flame temperature can be carefully controlled, diminishing the production of nitrogen oxide compounds (Nox), the levels of which are carefully monitored by government regulatory agencies. By extending combustion over two stages, the thermal peaks that produce NOx are moderated.
As with other types of burners, staged burners are varied from high fire to low fire in order to effect turndown. The previous burner of FIG. 1 includes a common mass flow ratio control system. The thermal demand of the system is linked to the flow of an independent reactant, which can be either the primary or secondary reactant. As thermal demand increases, the flow of the independent reactant is increased. The ratio control system varies the flow of the remaining dependent reactant, maintaining the respective reactant flows in the proper proportion. The ratio control system includes a control unit 38 which operates a motorized valve 34 for varying the flow of the common fuel supply 18. Similarly, air flow 12 is also varied using a motorized valve 36 controlled by the control unit 38. The primary and secondary flows 22, 24 are fixed by the respective limiting orifices 30, 32. Thus, the primary and secondary flows are supplied at rates which are in a fixed proportion to each other as flow is varied between high fire and low fire. This fixed proportion creates several problems in burner operation.
FIG. 2A illustrates the change in .phi. as a function of burner input during thermal turndown for a typical premixed air-staged control system. During high fire (100% input), air is supplied to the fuel flow in the primary stage as that the primary stage .phi. 42 runs at a particular rich ratio 40 (typically about 1.4). Additional air is added in the secondary stage so as to establish an overall burner .phi. 44 that is less than one, i.e. about 10% excess air (.phi.=0.909). During thermal turndown, the fuel supply 18 is lowered from 100% at a rate faster than the air supply 12. Since the proportion of air flow to each stage is fixed, the primary stage .phi. 42 decreases in proportion with the overall burner .phi. 44. At some point 46 during turndown, the primary stage will cross the stoichiometric ratio. At that point, the secondary stage is merely adding excess air and thus the benefits of staged combustion are lost.
FIG. 2B illustrates the change in .phi. as a function of burner input during thermal turndown for a typical premixed fuel-staged control system. (Of course, the systems described herein can also be nozzle-mixed systems. During high fire (100% input), fuel is supplied to the air flow in the primary stage so that the primary stage .phi. 52 runs at a particular lean ratio 50 (typically about 0.6) which is above the lean limit. Additional fuel is added in the secondary stage so as to establish an overall burner .phi. 54 that is less than one, i.e. about 10% excess air (.phi.=0.909). During thermal turndown, the fuel supply 18 is lowered from 100% at a rate faster than the air supply 12. Since the proportion of air flow to each stage is fixed, the primary stage .phi. 52 decreases in proportion with the overall burner .phi. 54. At some point 56 during turndown, the primary stage will cross the lean flammability limit for a premixed system, at which point the burner flame is extinguished. In view of these operational problems, the fixed reactant delivery through the limiting orifices of previous systems does not provide reliably effective thermal turndown.
There are several factors that also influence thermal input in previous systems even under constant firing with fixed reactant flows defined by the limiting orifices 30, 32. Air and fuel composition can vary over time, affecting the effective equivalence ratio. For example, cold air is more dense than hot air, and thus hot air has less oxygen per unit volume than cold air supplied at a comparable pressure. Hot air thus makes the burner fire rich. Some burner systems are operated under desert conditions where air temperatures can vary as much as 100.degree. F from night to day. Also, some systems use preheated air which may be quite hot and thus considerably less dense. Thus, air temperature can affect the equivalence ratio. Humidity can also affect the equivalence ratio since humid air has less oxygen content than dry air for a given volume, temperature and pressure. Thus, humid air also makes the burner fire rich.
Fuel composition can also vary over time, thus affecting the equivalence ratio. Natural gas supplies are derived from various sources and the calorific value of utility supply natural gas can vary by as much as 10% over time. Since most common burner systems use utility gas, the burner can vary between rich or lean firing depending on the composition of the fuel supply. Since the previous systems are limited to fixed reactant flows, none can compensate for the variations in the composition of air and fuel.
FIG. 3 illustrates a curve of optimal performance for a staged burner during preheated air operation. As preheated air temperature (T) is increased, the equivalence ratio .phi. in the primary stage must be decreased in order to maintain the optimum firing ratio 60. NOx production becomes a problem if the primary stage is operated at an equivalence ratio which is too high for a given thermal input. If the equivalence ratio is held constant with increasing preheat temperature, the mixture will fire rich, thereby increasing NOx production. Above a certain rich limit 62, the firing conditions 66 are such as will produce unacceptably high NOx.
As also seen in FIG. 3, if the primary equivalence ratio is held constant with decreasing air preheating, the mixture fires more lean, producing an unstable, inefficient flame, and possibly crossing the premix lean flammability limit 64 into conditions of flame extinction 68. Under these conditions, no flame occurs in the primary stage and the burner is shut down by the flame monitoring systems typically used with such burners.
As seen from FIG. 3, there is a narrow window of desirable operating conditions for variable air preheat conditions in a staged burner. However, previous systems are limited by fixed reactant flow proportions and are typically varied manually. It is not uncommon to operate staged burners at conditions which are not optimal or even acceptable. Thus, the previous systems do not offer adequate control over the equivalence ratios while using preheated air, thereby sacrificing the benefits of staged systems and producing unacceptable emission levels.
Another method often used with previous systems for controlling NOx production is Flue Gas Recirculation (FGR). With this technique, a portion of the burnt effluent from the burner output is drawn back and mixed with the air flow 12. FGR effects the energy balance of the burner, since recirculated flue gas, as an inert diluent, acts as an additional thermal load, thus lowering the temperature of the burner flame. The flame temperature is suppressed by an amount related to the percentage of flue gas recirculated into the air flow 12. Since flame temperatures are thereby suppressed, NOx emission are lowered.
While lowering NOx emissions, FGR tends to increase the lean flammability limit, thus driving up the equivalence ratio of the primary stage. FIG. 4 illustrates the variation in the equivalence ratio of the primary stage as a function of FGR, where FGR is measured as the ratio of FGR flow to combustion air flow. As FGR is increased, the equivalence ratio .phi. increases following an optimum firing ratio 80. If .phi. does not change with decreasing FGR, an operational limit 82 is reached, beyond which are conditions 86 of unacceptable NOx production. If .phi. does not change with increasing FGR, a lean limit 84 is reached, beyond which are conditions 88 of flame extinction. As is true with preheated air operation, the previous systems do not offer adequate control of the equivalence ratios for fluctuating conditions of FGR.